The present invention relates to pitch materials and more particularly to problems associated with stabilization of pitch material, either before or after the pitch material has been used to form fibers and other articles of manufacture.
Pitch is a graphitizable substance, i.e., a substance which fuses or becomes plastically deformed during heat treatment. Pitch is thermoplastic and thus can be melted by heating, followed by being allowed to cool to solidify, and then remelted and resolidified time and again. Pitch is a collection of hydrocarbons ranging from low molecular weight paraffins to high molecular weight large aromatics. There are byproduct pitches, such as coal tar or petroleum pitch, which are byproducts of oil cracking processes. There also are synthetic pitches. Two examples of synthetic pitches include specially prepared polyvinylchloride and specially prepared methylnapthalene. Mesophase pitch can be produced by such different techniques as heat treatment of coal tar or petroleum pitches or by solvent extraction of these pitches, or chemically prepared from methylnapthalene with the aid of HF/BF.sub.3. The coal tar and petroleum pitches are isotropic, while the mesophase pitches are anisotropic. Pyrolysis of pitch material, i.e., heating the pitch at temperatures above about 1,000.degree. C. in an inert atmosphere, converts the pitch to carbon or graphite material, depending on the parameters of the high temperature treatment. Generally, when talking about fibers, one produces carbon fibers at pyrolysis temperatures relative to about 1,700.degree. C. Above 1,700.degree. C., and on up to pyrolysis temperatures of 3,000.degree. C., the fibers are referred to as graphite fibers.
At room temperature in ambient air, pitch material exists as solid matter. It typically is supplied commercially in powdered or granular form or pellets, which are sized smaller than one cubic centimeter in volume. A common use of the pitch involves heating the pellets to about 350.degree. C., where they melt, and then extruding the melted pitch through tiny diameter holes to form elongated fibers having transverse cross-sectional diameters on the order of 10 to 100 microns. Desirably, these pitch fibers can be pyrolyzed (heat treated in an inert atmosphere at temperatures above 1,000.degree. C.) in order to form carbon and/or graphite fibers which have numerous industrial applications. A typical carbonization temperature is 1,500.degree. C., and a typical graphitization temperature is 2,400.degree. C. However, subjecting the pitch fibers immediately to temperatures above the 350.degree. C. melting point of the pitch, would melt the fibers back into an amorphous, free-flowing mass of pitch.
Accordingly, before pitch material can be pyrolyzed and transformed thereby into carbon and/or graphite material, the pitch must be heat stabilized in a manner that permits the pitch to maintain its shape and molecular orientation while undergoing pyrolysis. The thermoplastic pitch must be thermoset and rendered infusible so that it no longer melts when heated in the absence of oxygen. In other words, the heat stabilization process permits the pitch to retain the pitch's physical integrity and molecular integrity during the pyrolysis process.
The heat stabilization process has many names, such as stabilization, thermosetting, curing, oxidation, oxidative stabilization, etc. Whatever the name, the process used to stabilize the pitch involves gradually heating the pitch in an oxygen-containing atmosphere from room temperatures to temperatures just below the melting point of the pitch, typically about 350.degree. C. This can be accomplished by contacting the pitch material with heated flowing air. For example, a typical stabilization technique for mesophase pitch carbon fibers with diameters in the range of 10 to 40 microns, would begin heating at room temperature (25.degree. C.) in an air atmosphere to a temperature of 225.degree. C. at a constant rate for thirty minutes (6.67.degree. C./min.), followed by maintaining the 225.degree. C. temperature for an additional thirty minutes, and followed by heating from 225.degree. C. to 265.degree. C. at a constant rate over a period of 180 minutes (0.22.degree. C./min.).
Stabilization of isotropic pitch or mesophase pitch involves numerous chemical reactions coupled with mass transport of reactant, primarily oxygen, from the ambient air (or an artificially created oxygen-rich atmospheric environment) to reactive sites inside the pitch article. The oxygen that is absorbed, cross-links aromatic structures which preserve the axial preferred orientation of the article during pyrolysis. To avoid reorientation and rearrangement of the mesophase molecules, the stabilization heat treatment must be performed below the glass transition temperature. Authors have also reported that stabilization heat treatment at low temperatures favors the formation of carbonyls such as quinones and carboxylic acids, which produces homogeneous stabilization. Higher temperature stabilization heat treatments have resulted in ester cross links, and CO, and CO.sub.2 products after C/C bond fissure.
Infrared absorption (IR), thermogravimetry analysis (TGA), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), and various elemental analysis are techniques extensively used to study the oxidation mechanisms. A useful characterization of the stabilization of the pitch article can be related to the profile that is obtained by noting the lobalized concentration of oxygen at different depths measured from the surface of the article. For example, because of their size, stabilization of large diameter pitch-based fibers is rarely homogeneous in the cross section of the fibers. To homogeneously stabilize a fiber, oxygen needs to diffuse into the whole fiber. For very short oxidation times or for large diameter fibers, oxygen does not reach the center of the fiber, and such stabilization tends to introduce a skin/core microstructure during carbonization. The skin thickness is believed to vary as the square root of time, indicating a diffusion controlled process of stabilization. The outer part (nearest the outer surface of the fiber) often appears to be over oxidized, while the core (at the center of the fiber) is insufficiently stabilized and allows porosity formation and higher mobility of the molecular structure during pyrolysis. A fiber with such a profile of stabilization cannot be expected to have good mechanical properties. After carbonization, the skin is generally characterized by very fine sheets of crystallites, and coarse sheets of about 2.0 to 3.0 microns in width prevail in the core.
The chemical bonds created when the oxygen reacts with pitch, tend to increase the melting point of the pitch until the pitch has been rendered infusible. Increased penetration of oxygen into the interior of the pitch article promotes more stabilization. As the stabilization process progresses, the pitch material undergoes a decrease in solubility and an increase in mechanical properties such as tensile modulus.
The optimal stabilization process particulars, i.e., heating rates and durations, isothermal heating temperatures and durations, and maximum temperatures, of the stabilization process used for one application may vary somewhat from the optimal stabilization process particulars for a different application. The variance in process particulars may manifest itself in the rate of temperature increase and the duration of time spent during one or more temperature increases or at one or more discrete temperature levels. For example, the temperature of stabilization depends on the reactivity of the pitch material to be stabilized, which is a function of the material itself. Typical stabilization temperatures range from 220.degree. C. to 350.degree. C. Moreover, the required duration of the stabilization step depends on temperature, heating rate, the reactivity of the pitch, and the maximum depth which must be penetrated by the oxygen. As a general rule the thicker the article, the longer it takes to stabilize at any given temperature. Thus, larger diameter pitch fibers would be expected to require longer heating times. Heat treatments can take from a few minutes to two hours for small diameter fibers, and more than six hours can be expected for large diameter fibers. Indeed, most pitch articles with dimensions more than 100 microns thick cannot be economically stabilized because of the long time that the pitch must be maintained at temperatures on the order of 300.degree. C. and the inadequate stabilization of the innermost regions of the article.